Manufacture of semiconductors with high device densities requires the high-resolution lithography of small, closely-spaced features on semiconductor wafers. Such features are typically formed first by a photolithography process in temporary layers of photoresist, and the photoresist features are then used to create permanent structures on the wafer. For example, holes are formed in insulating layers and later filled with a conductive material to create connections between layers in a circuit. Trenches are also formed in insulating layers and later filled with a conductive material to form capacitors. Groups of thin conductive lines are formed to make buses to carry signals from one area of a chip to another. The groups of conductors are characterized by the width of each conductor and a pitch, that is, the distance between the conductors.
As device density increases, the difficulty of producing photo-masks for the lithographic process without defects also increases. Defects on photo-mask layers commonly arise from small particles of foreign material, bubbles in the photo-resist, or other flaws introduced during the pattern generation process. To replace a defective mask is impractical since a subsequently developed mask is as likely to be defective as the mask to be replaced. Thus, repair of a defective mask is more desirable. To this end, there is a need to constantly monitor the fabrication process to identify and repair defects. In some cases, every wafer going through the fabrication line is measured in what is sometimes referred to as in-line metrology.
Engineers may monitor both the features on the temporary photoresist layer and the permanent features created on the wafer. These features are three-dimensional structures and a complete characterization must describe not just a surface dimension, such as the top width of a hole or conductor, but a complete three-dimensional profile of the feature. For example, although an ideal feature typically has vertical sidewalls, the actual sidewalls may have excessive slope that narrows or widens the feature below its top surface. Process engineers must be able to accurately observe defects in order to repair them.
Methods of observation are known in the art. For example, scanning probe microscopy, allows 3-D imaging of surfaces with sub-nanometer resolution. A scanning probe microscope (SPM) uses a very small probe tip that is scanned at a slow rate across the surface of a substrate. There are many types of SPMs, including several types of atomic force microscopes (AFM). An SPM can operate in two different modes—contact mode or non-contact mode. In contact mode, the probe tip comes into physical contact with the sample surface. In non-contact mode, the probe tip does not actually touch the sample surface. Instead, the probe tip is in close proximity to the sample surface and interactive forces between the probe tip and the surface are measured.
In one type of AFM used in semiconductor processing, the probe tip is attached to a cantilever that is in turn attached to a piezoelectric actuator. The vibration amplitude of the cantilever driven near resonance frequency is monitored by reflecting a laser beam off the back surface of the cantilever into a photodiode sensor. Changes in the vibration amplitude of the cantilever cause changes in the readout signal of the optical sensor at the same frequency. The amplitude is sensitive to changes in tip-sample interaction, usually a result of distance change between tip and sample. The actuator maintains a level of tip-sample interaction. The probe tip moves up and down in response to peaks and valleys on the sample surface. The vertical positions at or near contact are tabulated and provide a profile of the surface.
A piezoelectric scanner (capable of extremely fine movements) typically is used as a positioning stage to accurately position the probe tip over the sample. The scanner moves the probe tip across the first line of the scan, and back. It then steps in the perpendicular direction to the second scan line, moves across it and back, then to the third line, and so forth. The path differs from a traditional raster pattern in that the alternating lines of data are not taken in opposite directions. AFM data are usually collected in only one direction to minimize line-to-line registration errors that result from scanner hysteresis.
As the scanner moves the probe tip along a scan line, the AFM collects data concerning the surface of the sample at equally spaced intervals. The spacing between the data points is called the step size or pixel size. The accuracy of the scan can be increased by using a smaller pixel size (which results in a greater number of data points, also referred to as pixel density). However, scans using a greater pixel density take longer to complete and require more resources to store and process.
Cantilever-based AFM instruments suffer from a number of shortcomings that limit their usefulness for mass-production CD metrology applications. Contact-mode AFMs, especially those where the probe tip remains in constant contact with the sample surface, are prone to tip wear and the gradual buildup of contamination from the sample onto the probe tip. Non-contact AFMs do not have a problem with tip wear, however, they are much more vulnerable to error caused by localized charges, humidity, or even particulate contamination. Both types of cantilever-based AFMs also have the potential for tip or sample damage because the tip has to stay so close to the sample surface. Even more significant, both types of cantilever-based AFMs suffer from low throughput. When operated at a sufficient resolution to accurately measure current critical dimensions, it can take several minutes to make measurements across one feature and several hours to measure a 50 μm square area. For these reasons, prior art AFMs in a production operation can profile only a limited area on a semiconductor chip.
One commonly used type of stylus device is a profilometer device such as the Stylus Nano-Profilometer (SNP), commercially available from FEI Company, Hillsboro, Oreg., the assignee of the present application. In contrast to the cantilever system employed by the typical AFM, the SNP makes use of a probe tip attached to a rocking balance beam, similar to those disclosed in U.S. Pat. No. 5,307,693, to Griffith et al. for “Force-Sensing System, Including a Magnetically Mounted Rocking Element” and in U.S. Pat. No. 5,756,887 to Bryson et al. for “Mechanism for Changing a Probe Balance Beam in a Scanning Probe Microscope.” Unlike other devices that sense while scanning the probe over the surface, the SNP measures the geometry of a surface by descending to the surface and contacting it at different points. Thus, unlike scanning devices, the SNP is designed to rapidly move downward toward a surface, contact the surface, and then move rapidly to a new position. Recently developed probe tips have a cylindrically or approximately square shaped cross section of dimensions of 0.2 μm or less. Such a small probe tip is usually relatively short, on the order of a micrometer, and is supported on a proximal end by a more massive tip support.
A typical probe is schematically illustrated in FIG. 1. During operation of the SNP, the probe 20 is discontinuously scanned horizontally along a line. At multiple positions, which are typically periodically spaced, the horizontal scan of the probe 20 is stopped, and it is lowered until it is stopped by the substrate surface 12. Circuitry to be briefly described later measures the height at which the probe tip stops. The SNP probe 20 is then retracted from the surface 12 a distance sufficient to clear any vertical features, moved horizontally a preset distance, and then moved vertically back toward the surface 12. A series of such measurements around the feature being probed, for example, on either sidewall 14 of and within the trench 10, provides a profile or topography of the sample.
An SNP is schematically illustrated in the side view of FIG. 2. A sample 30 to be examined is supported on a support surface 32 supported successively on a tilt stage 34, an x-slide 36, and a y-slide 38, all of which are movable along their respective axes so as to provide horizontal two-dimensional and tilt control of the sample 30. The tilt stage is also capable of 10 mm or more of vertical Z travel. Although these mechanical stages provide a relatively great range of motion, their resolutions are relatively coarse compared to the resolution sought in the probing. The bottom y-slide 38 rests on a heavy granite slab 40 providing vibrational stability. A gantry 42 is supported on the granite slab 40. A probe head 44 hangs in the vertical z-direction from the gantry 42 through an intermediate piezoelectric actuator 45 that provides about 30 μm of motion in x and y and 15 μm in z by way of a piezo driven 3-axis flexure system 46 controlled by linearized capacitors in closed loop. Probe head 44 includes a small attached probe 20 that projects downwardly from the probe head 44 so that probe 20 can selectively engage with the top surface of the sample 30 and thereby determine its vertical and horizontal dimensions.
Principal parts of the probe head 44 of FIG. 2 are illustrated in the side views of FIGS. 3 and 4. A dielectric support 50 fixed to the bottom of the piezoelectric actuator 45 includes on its top side, with respect to the view of FIG. 2, a magnet 52. On the bottom of the dielectric support 50 are deposited two isolated capacitor plates 54, 56 and two interconnected contact pads 58. A conductive beam 60 is medially fixed on its two lateral sides and electrically connected to two metallic and ferromagnetic ball bearings 62, 64. Ball bearings 62, 64 are placed on the contact pads 58 and generally between the capacitor plates 54, 56, and held in place by magnet 52.
Beam 60 is held in a position generally parallel to the dielectric support 50 with a balanced vertical gap 66 of about 25 μm between the capacitor plates 54, 56 and the beam 60. Two capacitors are formed between the respective capacitor pads 54, 56 and the conductive beam 60. Capacitor pads 54, 56 and contact pads 58, all electrically connected to the conductive beam 60, are also connected to three terminals of external measurement and control circuitry to be described later. Beam 60 holds on its distal end a glass tab 70 to which is fixed a stylus 72 having a probe 20 projecting downwardly to selectively engage the top of the sample 30 being probed. An unillustrated dummy stylus or substitute weight on the other end of the beam 60 can provide rough mechanical balancing of the beam in the neutral position.
The typical SNP operates through the use of a force-balance system, so that externally applied force (such as the force generated when the probe tip encounters a feature) acts on a sensing device, the output of which produces a locally generated counteracting force to drive the sensor output back to zero. Capacitor plates 54, 56 and the two contact pads 58 are separately connected by three unillustrated electrical lines to three terminals of external measurement and control circuitry. This servo system both measures the two capacitances and applies differential voltage to the two capacitor plates 54, 56 to keep them in the balanced position. When the piezoelectric actuator 45 lowers the stylus 72 to the point that probe 20 encounters the surface of the feature being probed, the beam 60 rocks upon the contact of probe 20 with the sample 30. The difference in capacitance between the plates 54, 56 is detected, and the servo circuit attempts to rebalance the beam 60 by applying different voltages across the two capacitors.
Two different feedback loops are thus at work during the operation of the SNP. When the probe head 44 is lowered until the probe 20 touches the sample surface 30, the balance beam 60 will typically be knocked a little off balance. The dual feedback system works to adjust the vertical position of the probe 20 by way of the piezoelectric actuator 45 and to bring the beam 60 back into balance by applying different voltages across the two capacitors. Whenever the force acting on the probe 20 (measured by the voltage applied in order to balance the beam) is below a certain force set point, the piezoelectric actuator 45 will lower the probe head. If the force acting on the probe 20 is above the force set point, the piezoelectric actuator 45 will raise the probe head 44. When the two feedback loops reach equilibrium, the force acting on the probe 20 will be at the force set point and the beam 60 will be returned to a balanced state. When this occurs, the vertical position of the piezoelectric actuator 45 can be used as an indication of the depth or height of the feature at the particular data point being measured.
In some applications, the SNP uses optical pattern recognition to locate the features of interest to within approximately 1 micron. Therefore, it is necessary to establish the offset between the optical system and the tip. This is done by scanning a pattern called a “tipfinder” after locating a reference point on the tipfinder with the optical system. The tipfinder is described in U.S. Pat. No. 6,178,653. A scan line anywhere on the area of the pattern is coded in bits vertically to the X, Y coordinate of the pattern, thus establishing where the tip is relative to the optical system.
Methods of repair are also known in the art including focused ion beam milling and laser ablation. However, repair of small defects using these methods is very difficult. Alternatively, repair of small defects can be made using a scanning tunneling microscope probe to observe features and to scrape off excess material from the mask or wafer and to deposit material where material is deficient. This method is described in U.S. Pat. No. 6,197,455. However, this method requires interaction between the probe and a conducting surface for observation of the features to be repaired.
Repair using scanning probe microscopy is generally discussed in U.S. Pat. No. 6,353,219, wherein it is suggested that a cantilever-mounted probe can be pressed against the surface and dragged across the surface to cut into it. Cantilever systems, which are designed to scans above a surface, must approach the surface slowly to prevent probe tip damage. This slows processing when the probe must be moved between calibration structures and the work piece or when switching between measuring and processing. Also, in the system therein described, the force and more importantly the pressure applied by the probe to the surface may vary as the probe is scanned over different topography. Since wear is related non-linearly to force, wear is unpredictable over time.
What is needed is a method of repair of defects in features of a mask or wafer that may be conductive or non-conductive. What is also needed is to combine such a method with a method of observation implemented by a single instrument. Further, what is needed is a method for performing repair of defects using multiple rapid strokes in a definable pattern at known high applied pressure.